The present invention is directed to printheads useful for ink jet printing processes. More specifically, the present invention is directed to printheads having improved ink repellency on the front faces or nozzle plates thereof. One embodiment of the present invention is directed to an ink jet printhead comprising a plurality of channels, wherein the channels are capable of being filled with ink from an ink supply and wherein the channels terminate in nozzles on one surface of the printhead, said surface having covalently bonded thereto a coating of an organosiloxane polymer, said organosiloxane polymer coating being substantially uniform with no additional layers thereover. Another embodiment of the present invention is directed to a process for preparing a printhead suitable for ink jet printing which comprises (a) providing an ink jet printhead comprising a plurality of channels, wherein the channels are capable of being filled with ink from an ink supply and wherein the channels terminate in nozzles on one surface of the printhead; (b) applying to said surface a coating of a composition comprising an organosiloxane polymer precursor material; and (c) exposing said organosiloxane precursor material to ultraviolet radiation, thereby causing polymerization, chain extension, and/or crosslinking of the precursor material and covalent bonding of the polymerized, chain extended, and/or crosslinked organosiloxane polymer thereby formed to the surface, said polymerized, chain extended, and/or crosslinked organosiloxane polymer coating being substantially uniform with no additional layers thereover. Yet another embodiment of the present invention is directed to a printing process which comprises (1) providing an ink jet printer containing a printhead comprising a plurality of channels, wherein the channels are capable of being filled with ink from an ink supply and wherein the channels terminate in nozzles on one surface of the printhead, said surface having covalently bonded thereto a coating of an organosiloxane polymer, said organosiloxane polymer coating being substantially uniform with no additional layers thereover; (2) incorporating into the printer an ink composition; and (3) causing droplets of the ink to be ejected in an imagewise pattern onto a recording sheet to form an image.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a-fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies.
Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, "Focusing Ink Jet Head," IBM Technical Disclosure Bulletin, Vol 16, No. 4, Sept. 1973, pp. 1168-1170, the disclosure of which is totally incorporated herein by reference, describes an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture was used to propel ink droplets out through a small ejection orifice. Acoustic ink printers typically comprise one or more acoustic radiators for illuminating the free surface of a pool of liquid ink with respective acoustic beams. Each of these beams usually is brought to focus at or near the surface of the reservoir (i.e., the liquid/air interface). Furthermore, printing conventionally is performed by independently modulating the excitation of the acoustic radiators in accordance with the input data samples for the image that is to be printed. This modulation enables the radiation pressure which each of the beams exerts against the free ink surface to make brief, controlled excursions to a sufficiently high pressure level for overcoming the restraining force of surface tension. That, in turn, causes individual droplets of ink to be ejected from the free ink surface on demand at an adequate velocity to cause them to deposit in an image configuration on a nearby recording medium. The acoustic beam may be intensity modulated or focused/defocused to control the ejection timing, or an external source may be used to extract droplets from the acoustically excited liquid on the surface of the pool on demand. Regardless of the timing mechanism employed, the size of the ejected droplets is determined by the waist diameter of the focused acoustic beam. Acoustic ink printing is attractive because it does not require the nozzles or the small ejection orifices which have caused many of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have suffered. The size of the ejection orifice is a critical design parameter of an ink jet because it determines the size of the droplets of ink that the jet ejects. As a result, the size of the ejection orifice cannot be increased, without sacrificing resolution. Acoustic printing has increased intrinsic reliability because there are no nozzles to clog. As will be appreciated, the elimination of the clogged nozzle failure mode is especially relevant to the reliability of large arrays of ink ejectors, such as page width arrays comprising several thousand separate ejectors. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink jet printing, including inks having higher viscosities and inks containing pigments and other particulate components. It has been found that acoustic ink printers embodying printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned pixels (i.e., picture elements) at resolutions which are sufficient for high quality printing of relatively complex images. It has also has been discovered that the size of the individual pixels printed by such a printer can be varied over a significant range during operation, thereby accommodating, for example, the printing of variably shaded images. Furthermore, the known droplet ejector technology can be adapted to a variety of printhead configurations, including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from single row, sparse arrays for hybrid forms of parallel/serial printing to multiple row staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing. Inks suitable for acoustic ink jet printing typically are liquid at ambient temperatures (i.e., about 25.degree. C.), but in other embodiments the ink is in a solid state at ambient temperatures and provision is made for liquefying the ink by heating or any other suitable method prior to introduction of the ink into the printhead. Images of two or more colors can be generated by several methods, including by processes wherein a single printhead launches acoustic waves into pools of different colored inks. Further information regarding acoustic ink jet printing apparatus and processes is disclosed in, for example, U.S. Pat. Nos. 4,308,547, 4,697,195, 5,028,937, 5,041,849, 4,751,529, 4,751,530, 4,751,534, 4,801,953, and 4,797,693, the disclosures of each of which are totally incorporated herein by reference. The use of focused acoustic beams to eject droplets of controlled diameter and velocity from a free-liquid surface is also described in J. Appl. Phys., vol. 65, no. 9 (1 May 1989) and references therein, the disclosure of which is totally incorporated herein by reference.
Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
"Plasma Deposition of Thin Films from a Fluorine-Containing Cyclosiloxane," P. Favia et al., Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 32, 121-130 (1994), the disclosure of which is totally incorporated herein by reference, discloses the deposition of thin films from radio-frequency glow discharges fed with vapors of a silicon- and fluorine-containing organic compound, namely 2,4,6-tris((3,3,3-trifluoropropyl)(methyl))cyclotrisiloxane, of the formula ##STR1##
in mixture with argon. A triode reactor was used to deposit films by independently changing substrate temperature and bias-induced ion-bombardment. Laser interferometry, electron spectroscopy for chemical analysis, and Fourier-transform infrared spectroscopy were used to monitor film growth rate and composition. The results showed an activating effect of the ion-bombardment. Low substrate temperature and bias conditions resulted in films with a "monomer-like" stoichiometry, while drastic conditions gave origin to materials with a completely different composition and a markedly increased hardness.
"Laser-induced Generation of Thin Silicone Layers with High Chemical and Spectral Purity," W. Roth et al., Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 32, 1893-1898 (1994), the disclosure of which is totally incorporated herein by reference, discloses the use of excimer lasers (ArF, .lambda.=193 nm, and KrF, .lambda.=248 nm) to generate polymers free of additives such as catalysts, initiators, or sensitizers. The layers obtained were of potential interest for medical applications and future molecular electronics. Dimethylpolysiloxanes and dimethylsiloxane copolymers containing phenyl-, n-hexyl-, or 3,3,3-trifluoropropyl groups or silicon-bound hydrogen atoms were crosslinked in the liquid phase, whereby layer thicknesses in the range from 1 to 300 microns were obtained. Disiloxanes and alkoxysilanes were deposited from the gas phase (laser chemical vapor deposition), resulting in layer thicknesses below 1 micron. In almost all cases, organic layers with a smooth surface, transparency, and good adhesion were obtained on silicon as well as quartz substrates.
"Silicones in the UV/EB Coatings Industry: Influence of Chemical Structure on Performance," E. Orr, Journal of Radiation Curing, Vol. 22, No. 1, 13-19 (1995), the disclosure of which is totally incorporated herein by reference, discloses an analysis of silicones with special emphasis on polyether-modified and polyester-modified polysiloxanes. The chemical determinants of silicone performance are outlined for UV/EB coatings, inks, adhesives, and related applications. Structure-performance correlations, system compatibility, surface tension effects, thermostability, wetting/leveling, and slip/mar resistance are also discussed.
"Excimer Laser Photolysis of Metalorganic Complexes of Platinum and Palladium in the Gas Phase," H. Willwohl et al., Appl. Surf. Sci., Vol. 54, 89-94 (1992), the disclosure of which is totally incorporated herein by reference, discloses the KrF-excimer-laser-photolysis (248 nm) of the bishexafluoroacetylacetonates of platinum and palladium in the gas phase. Platinum bishexafluoroacetylacetonates are identified as precursors in laser chemical vapor deposition.
"Deposition of High Quality SiO.sub.2 Layers from TEOS by Excimer Laser," A. Klumpp et al., Appl. Surf. Sci, Vol. 36, 141-149 (1989), the disclosure of which is totally incorporated herein by reference, discloses the deposition of SiO.sub.2 layers on silicon wafers from a mixture of tetraethylorthosilicate and oxygen by ArF-excimer laser radiation. The deposition conditions were studied as a function of substrate temperature, partial pressure, and laser fluency. Deposition rates as high as 2,000 .ANG./min at pulse energies of 100 mJ/cm.sub.2 were obtained. The physical properties of the SiO.sub.2 layers were investigated by FT-IR spectroscopy, Rutherford backscattering, and ellipsometry. The electrical properties of breakdown voltage, interface state density, and mobile-ion density are also given. The SiO.sub.2 layers show nearly the same quality as thermally grown SiO.sub.2 layers.
U.S. Pat. No. 5,212,496 (Badesha et al.), the disclosure of which is totally incorporated herein by reference, discloses an ink jet recording head comprising a plurality of channels, wherein the channels are capable of being filled with ink from an ink supply and wherein the channels terminate in nozzles on one surface of the printhead, the surface being coated with a polyimide-siloxane block copolymer.
U.S. Pat. No. 5,121,134 (Albinson et al.), the disclosure of which is totally incorporated herein by reference, discloses a method of providing the surface area of a substrate with a first zone which is solvent wettable and a second zone which is solvent nonwettable, and which is particularly suitable for application to the printheads and nozzle plates of drop-on-demand ink jet printers or like products where the spacing between zones of the same kind can be as little as just tens of microns, and wherein the solvent nonwettable zone displays excellent abrasion resistance and resistance to solvents, is virtually nonwettable by a wide range of solvents, and bonds well even to plastic substrates. The method comprises (1) providing a surface having good solvent wettability at least over that part of the area of the substrate which is to form the first zone; (2) providing the area with a first layer which comprises siloxic material which bonds to the substrate and which is in contact with the substrate over at least that part of the area which is to form the second zone; (3) providing the area with an overlayer comprising organic fluorocompound which bonds to the first layer and provides a surface of poor solvent wettability, said overlayer being in contact with the first layer over at least that part of the area which is to form the second zone; and (4) by etching or washing, removing overlying material from the surface having good solvent wettability over that part of the area which is to form the first zone whereby to expose said surface.
British Patent Document GB 8824436 A0, the disclosure of which is totally incorporated herein by reference, discloses a method of reducing the wettability of non-vitreous surfaces, and ink jet recording heads including a surface having reduced wettability, wherein a layer of cured siloxane is formed on the non-vitreous surface and a layer derived from at least one fluorosilane is formed on the siloxane layer.
While known compositions and processes are suitable for their intended purposes, a need remains for improved ink jet printheads. In addition, a need remains for ink jet printheads having front faces or nozzle plates with improved ink repellency. Further, a need remains for ink jet printheads with ink repellent coatings that are abrasion resistant and do not wear off rapidly under the action of a wiper blade typically employed in the maintenance station of a ink jet printer. Additionally, a need remains for ink jet printheads having ink repellent coatings that can be deposited onto the nozzle plate or front face without being deposited in or on the ink channels. There is also a need for ink jet printheads having ink repellent coatings on the front faces or nozzle plates thereof, wherein the coatings adhere well to the printheads. In addition, a need remains for ink jet printheads having ink repellent coatings on the front faces or nozzle plates thereof, wherein the coatings are mechanically strong and resistant to abrasion. Further, a need remains for processes for preparing improved ink jet printheads. Additionally, a need remains for processes for modifying the surface characteristics of the front faces or nozle plates of ink jet printheads. There is also a need for processes for modifying the surface characteristics of the front faces or nozzle plates of ink jet printheads by applying ink repellent coatings or layers thereon, wherein the thickness of the coating or layer can be controlled. In addition, there is a need for ink jet printheads having ink repellent coatings or layers on the front faces or nozzle plates thereof that exhibit good adhesion and abrasion resistance when subjected to cleaning or wiping. Further, there is a need for ink jet printheads having relatively thick ink repellent coatings or layers on the front faces or nozzle plates thereof. Additionally, there is a need for ink jet printheads having ink repellent coatings or layers on the front faces or nozzle plates thereof that are covalently bonded to the printhead.